Put potatoes in medium sauce pan and cover with water. Cook over medium high heat until tender and drain. Add potatoes, sugar, eggs, milk, salt, vanilla and butter to a mixer, beat until smooth. Grease 2-quart casserole dish and add potato mixture. In medium bowl, mix together brown sugar and flour. Cut in butter until mixture is coarse. Stir in pecans. Sprinkle mixture over sweet potato mixture. Bake 30 minutes until bubbly and topping is brown.